A sensor which particularly senses fingerprints is reported as a sensor for recognizing a surface shape having fine ridges and valleys. Also, as a technique for detecting fingerprint patterns, a capacitive fingerprint sensor using the LSI fabrication technique is proposed. Examples of the capacitive fingerprint sensor are described in reference 1 (Japanese Patent Laid-Open No. 2000-346608) and reference 2 (“A Robust, 1.8 V 250 μW Direct-Contact 500 dpi Fingerprint Sensor”, ISSCC DIGEST OF TECHNICAL PAPERS, February 1998, pp. 284-285).
As shown in FIG. 18, each of these capacitive fingerprint sensors is formed as a sensor cell array 2 in which sensor cells 1 are two-dimensionally arrayed on an LSI chip, and detects the capacitance formed between a sensor electrode of each sensor cell 1 and the skin of a finger 3 which comes in contact with the sensor electrode via an insulating passivation film, thereby sensing the pattern of ridges and valleys of the fingerprint. Since the value of the capacitance changes in accordance with a ridge or valley of a fingertip skin surface, a ridge or valley of a fingertip skin surface can be sensed by detecting this fine capacitance difference.
As shown in FIG. 19, a sensor electrode 101 is incorporated into each sensor cell 1 of the sensor cell array 2.
A surface shape recognizing sensor device as the first prior art using the principle of the capacitive fingerprint sensor shown in FIG. 18 will be explained with reference to FIG. 20. In the surface shape recognizing sensor device shown in FIG. 20, each sensor cell 1 comprises a detecting element 10, signal generating circuit 11, switch SW1, and detection circuit 12.
The detecting element 10 includes an insulating layer 100 on a substrate, a sensor electrode 101 formed on the insulating layer 100, and a passivation film 102 so formed as to cover the sensor electrode 101.
The signal generating circuit 11 includes a switch SW2 which generates a voltage signal corresponding to a capacitance Cf formed between the sensor electrode 101 and the skin of a finger 3 in contact with the passivation film 102, and a current source 110. The detection circuit 12 detects the voltage signal from the signal generating circuit 11. The switch SW1 supplies a potential Vp to a node N1 as a connecting point between the sensor electrode 101 of the detecting element 10 and the output terminal of the signal generating circuit 11. Note that Cp in FIG. 20 denotes a parasitic capacitance.
Since the capacitance Cf is determined by the distance between the sensor electrode 101 and the skin of the finger 3, the value of Cf changes in accordance with a ridge or valley of a fingerprint. Accordingly, a voltage signal corresponding to a ridge or valley of the finger 3 is output from the signal generating circuit 11 to the node N1. This voltage signal is detected as a signal reflecting the ridge or valley of the fingerprint by the detection circuit 12, and as a consequence the fingerprint pattern is detected.
A normal operation of the surface shape recognizing sensor device shown in FIG. 20 will be explained with reference to FIGS. 21A to 21D. The surface of the finger 3 is connected to the ground potential (GND) via a resistance Rf of the finger 3. Assume that Rf=0Ω. Accordingly, the potential of the finger surface, i.e., the potential at a node N2 is held at the ground potential (FIG. 21D).
Initially, a control signal P for controlling opening/closure of the switch SW1 is Low level (FIG. 21A). A control signal S1 for controlling opening/closure of the switch SW2 is also Low level (FIG. 21B). Therefore, both the switches SW1 and SW2 are open. In this case, the potential at the node N1 is equal to or lower than the potential Vp (FIG. 21C).
In this state, if the control signal P changes from Low level to High level at time t1 in FIG. 21A, the switch SW1 is closed and turned on, and consequently the potential at the node N1 is precharged to the potential Vp (FIG. 21C).
After the precharge is completed, the control signal P changes to Low level at time t2 in FIG. 21A, and simultaneously the control signal S1 changes to High level as shown in FIG. 21B. Accordingly, the switch SW1 is turned off, the switch SW2 is turned on, and the electric charge stored in the node N1 is extracted by the current source 110. As a consequence, the potential (voltage signal) at the node N1 lowers (FIG. 21C). Letting Δt be a High-level period of the control signal S1, a potential drop ΔV of the node N1 from the potential Vp when Δt has elapsed is given byΔV=IΔt/(Cf+Cp)   (1)where I is the current value of the current source 110, and Cp is a parasitic capacitance.
Since the electric current I, period Δt, and parasitic capacitance Cp are constant, the potential drop ΔV is determined by the capacitance Cf. The capacitance Cf is determined by the distance between the sensor electrode 101 of the detecting element 10 and the skin of the finger 3, so the value of the capacitance Cf changes in accordance with a ridge or valley of a fingertip skin surface. Accordingly, the change in magnitude of the potential drop ΔV reflects a ridge or valley of a fingertip skin surface. That is, letting Cfv be the capacitance formed between a valley of a fingertip skin surface and the sensor electrode 101 and Cfr be the capacitance formed between a ridge of a fingertip skin surface and the sensor electrode 101, a difference ΔVi between a voltage signal corresponding to a valley of a fingertip skin surface and a voltage signal corresponding to a ridge of a fingertip skin surface is given byΔVi=IΔt/(Cfv+Cp)−IΔt/(Cfr+Cp)   (2)Since, therefore, the voltage signal detected by the detection circuit 12 of each sensor cell is a signal reflecting a ridge or valley of a fingertip skin surface, ridges and valleys of a fingertip skin surface can be discriminated by outputs from a plurality of sensor cells.
The surface of the finger 3, however, is connected to the ground potential via the resistance Rf of the finger 3, so no sufficiently large voltage difference ΔVi can be obtained in some cases if the resistance Rf is high because, e.g., the finger 3 is dry. The operation of the surface shape recognizing sensor device when Rf>>0 will be explained with reference to FIGS. 22A to 22D.
The basic operation timings in FIGS. 22A to 22D are the same as in FIGS. 21A to 21D. On a ridge of a fingerprint, however, the potential of the finger surface, i.e., the potential at the node N2 cannot hold the ground potential and fluctuates as shown in FIG. 22D with the potential change at the node N1 shown in FIG. 22C. Consequently, the value of the capacitance Cf formed between the ridge of the fingertip skin surface and the sensor electrode 101 effectively decreases (Cf=αCfr, α<1), and as a result the voltage difference ΔVi (=IΔt/(Cfv+Cp)−IΔt/(α·Cfr+Cp)) decreases as shown in FIG. 22C. This makes it difficult for the surface shape recognizing sensor device shown in FIG. 20 to discriminate between the ridge and valley patterns of a fingerprint image, and consequently no clear fingerprint image pattern can be obtained.
A surface shape recognizing sensor device as the second prior art using the principle of the capacitive fingerprint sensor shown in FIG. 18 will be explained with reference to FIG. 23.
This surface shape recognizing sensor device differs from the example shown in FIG. 20 in the arrangement of a signal generating circuit 13. The signal generating circuit 13 includes a switch SW3 which selects and outputs a power supply potential VDD or ground potential GND, and a capacitive element Cs formed between the output terminal of the switch SW3 and a node N1. The signal generating circuit 13 extracts an electric charge from the node Ni by charging/discharging the capacitive element Cs, and the charge amount to be extracted is controlled by the capacitance value of Cs and a driving voltage Vs of Cs. In this device, the charge amount to be extracted from the node N1 is controlled by setting the driving voltage Vs shown in FIG. 23 at the power supply potential VDD (VDD>0) or ground potential GND via the switch SW3.
A normal operation of the surface shape recognizing sensor device shown in FIG. 23 will be explained with reference to FIGS. 24A to 24D. The surface of a finger 3 is connected to the ground potential via a resistance Rf of the finger 3. Assume that Rf=0Ω. Accordingly, the potential of the finger surface, i.e., the potential at a node N2 is held at the ground potential (FIG. 24D).
At time t1 in FIG. 24A, the switch SW1 is closed by changing the potential of a control signal P to High level, thereby precharging a potential Vp in the node N1. In this case, the driving voltage Vs of the capacitive element Cs in the signal generating circuit 13 is set at VDD. After that, at time t2 in FIG. 24A, the switch SW is opened by changing the potential of the control signal P to Low level. At the same time, as shown in FIG. 24B, the driving voltage Vs of the capacitive element Cs is decreased by ΔVs from VDD and set at GND, thereby extracting the electric charge from the node N1 to generate a voltage signal to a detection circuit 12.
A change amount ΔV of the voltage signal to be applied to the detection circuit 12 is give byΔV=ΔVs/{1+(Cf+Cp)/Cs}  (3)
A difference ΔVi between a voltage signal corresponding to a valley of a fingertip skin surface and a voltage signal corresponding to a ridge of a fingertip skin surface is given byΔVi=ΔVs/{1+(Cfv+Cp)/Cs}−ΔVs/{1+(Cfr+Cp)/Cs}  (4)Since, therefore, the voltage signal detected by the detection circuit 12 of each sensor cell is a signal reflecting a ridge or valley of a fingerprint, ridges and valleys of a fingertip skin surface can be discriminated by outputs from a plurality of sensor cells.
The surface of the finger 3, however, is connected to the ground potential via the resistance Rf of the finger 3, so no sufficiently large voltage difference ΔVi can be obtained in some cases if the resistance Rf is high because, e.g., the finger 3 is dry. The operation of the surface shape recognizing sensor device when Rf>>0 will be explained with reference to FIGS. 25A to 25D.
The basic operation timings in FIGS. 25A to 25D are the same as in FIGS. 24A to 24D. On a ridge of a fingertip skin surface, however, the potential of the finger surface, i.e., the potential at the node N2 cannot hold the ground potential and fluctuates as shown in FIG. 25D with the potential change at the node N1 shown in FIG. 25C. Consequently, the value of the capacitance Cf formed between the ridge of the fingertip skin surface and the sensor electrode 101 effectively decreases (Cf=αCfr, α<1), and as a result the voltage difference ΔVi (=ΔVs/{1+(Cfv+Cp)/Cs}−ΔVs/{1+(α·Cfr+Cp)/Cs}) decreases as shown in FIG. 25C. This makes it difficult for the surface shape recognizing sensor device shown in FIG. 23 to discriminate between the ridge and valley patterns of a fingerprint image, and as a consequence no clear fingerprint image pattern can be obtained.